1. Field of the Invention
Exemplary aspects of the present invention generally relate to a cleaning device capable of maintaining long-lasting cleaning performance and an image forming apparatus including the cleaning device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a transfer member (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a photoconductor; an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of transfer members; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
Toner having a smaller particle diameter and a round particle shape is now widely used in image forming apparatuses to meet increasing demand for higher-quality images. The smaller particle diameter can provide highly accurate and delicate images with higher resolution, and the round particle shape can achieve improved developing and transfer properties.
However, use of such toner prevents a well-known cleaning blade system from reliably cleaning the toner due to the following reasons.
In the cleaning blade system, a cleaning blade contacts a surface of an image carrier to scrape off the toner from the image carrier. At this time, the leading edge of the cleaning blade is deformed due to frictional resistance between the cleaning blade and the surface of the image carrier, resulting in stick-slip motion. Consequently, a minute space is generated between the cleaning blade and the image carrier.
The toner having the smaller particle diameter easily enters the minute space thus generated between the cleaning blade and the image carrier. In addition, when the toner entering the minute space has the round particle shape, torque tends to be generated at the toner, thereby rolling the toner within the minute space. As a result, smaller-diameter, round-particle toner lifts the cleaning blade. Consequently, the toner further easily enters the minute space between the cleaning blade and the image carrier. Thus, it is difficult to perform reliable cleaning of the toner using the well-known cleaning blade system.
One example of a method that can reliably clean even smaller-diameter, round-particle toner is an electrostatic cleaning method.
In the electrostatic cleaning method, a voltage having a polarity opposite a charging polarity of the toner is applied to a cleaning member such as a conductive cleaning brush contacting the image carrier, to electrostatically remove the toner from the image carrier.
However, any variation in the electric charge of untransferred toner conveyed to the cleaning member prevents the electrostatic cleaning method from reliably removing the toner from the image carrier, as described in detail below.
Much of the toner on the image carrier prior to transfer from the image carrier onto a transfer member such as a sheet of paper or other recording media is charged to a normal charging polarity of the toner, that is, a negative polarity. At a transfer position where the toner is transferred from the image carrier onto the sheet, a transfer magnetic field having a positive polarity opposite the normal charging polarity of the toner is applied to the toner borne on the image carrier to transfer the toner onto the sheet. However, a slight amount of toner remains attached to the image carrier after passing through the transfer position as untransferred toner.
The electric charge of the untransferred toner is shifted to the positive polarity due to the positive electric charge injected into the toner at the transfer position. Therefore, the untransferred toner remaining attached to the image carrier has a broad charging distribution having both the positively charged toner and the negatively charged toner.
However, in the electrostatic cleaning method described above, a positive voltage having a polarity opposite the normal charging polarity of the toner is applied to the cleaning brush as described above to electrostatically remove the toner from the image carrier. Consequently, it is difficult to remove the positively charged toner contained in the untransferred toner from the image carrier.
There is known a cleaning device in which a conductive blade is provided upstream from multiple cleaning brushes. The conductive blade contacts the image carrier and serves as a polarity controller that controls the charging polarity of the toner. A voltage having a polarity opposite a polarity of a voltage applied to a first cleaning brush is applied to the conductive blade.
Electric charges are injected into the untransferred toner from the conductive blade when the untransferred toner passes a contact position where the conductive blade contacts the image carrier. As a result, the untransferred toner is given the same charging polarity (usually the normal charging polarity of the toner) as the polarity of the voltage applied to the conductive blade.
Therefore, the untransferred toner that passes through the contact position and is further conveyed to the first cleaning brush has the same polarity as the polarity of the conductive blade, where it is electrostatically collected by the first cleaning brush to which a voltage having a polarity opposite the polarity of the voltage applied to the conductive blade is applied.
In the above-described example of the cleaning device, normally charged toner (e.g., negatively charged toner) on the image carrier is electrostatically attracted to the first cleaning brush serving as a normally charged toner cleaning member and is removed from the image carrier, and reversely charged toner (e.g., positively charged toner) on the image carrier is electrostatically attracted to a second cleaning brush serving as a reversely charged toner cleaning member and is removed from the image carrier.
As a result, both the positively and negatively charged toner can be removed from the image carrier.
However, in a case in which a toner pattern is formed on the image carrier to adjust an image density or to correct color shift, the image density is detected by a photosensor. After the detection of the image density, the toner pattern, which contains a larger amount of toner, is not transferred onto a sheet but is simply removed from the image carrier by the cleaning device.
In addition, in a case in which toner is consumed to replenish a developing device with new toner or irregular conveyance of the sheet causes sheet jam, a toner image containing a larger amount of toner formed on the image carrier is not transferred onto the sheet but is simply removed from the image carrier by the cleaning device.
Thus, the cleaning device removes the untransferred toner image such as the toner pattern containing a larger amount of toner, as well as the untransferred toner, from the image carrier.
However, the above-described related-art cleaning device cannot give a single charging polarity to the larger amount of toner contained in the untransferred toner image using the polarity controller. Consequently, toner having the same polarity as the polarity of the voltage applied to each of the cleaning brushes is conveyed to the respective cleaning brushes.
In addition, the larger amount of toner contained in the untransferred toner image may not be electrically attracted to the cleaning brushes. Consequently, the untransferred toner image is not reliably removed from the image carrier.
However, an electrostatic force in a repulsive direction relative to the normally charged toner acts on the second cleaning brush to which the negative voltage having the same polarity as the normal charging polarity of the toner is applied. Consequently, although the second cleaning brush has the higher ability to mechanically remove the toner from the image carrier, the bristles of the second cleaning brush do not contact the toner. As a result, some of the normally charged toner passes between the bristles of the brush, thereby preventing sufficient mechanical removal of the toner from the image carrier.
In addition, sometimes there is more positively charged toner than negatively charged toner contained in the untransferred toner. In such a case, the second cleaning brush having the smaller diameter and the lower ability to electrostatically remove the positively charged toner may not reliably remove the positively charged untransferred toner from the image carrier.
Further, there is also increasing demand for image forming apparatuses suitable for high-volume mass printing at reduced cost as well as higher quality images. In order to meet this demand, processing speed is increased, occurrence of downtime during maintenance or the like is reduced, and product life of consumable components is extended.
In a case of use of smaller-diameter, round-particle toner in the related-art cleaning blade system, the cleaning blade is pressed against the image carrier with a greater pressure to prevent the toner from entering the minute space between the cleaning blade and the image carrier. However, the larger load applied both to the cleaning blade and the image carrier due to the greater pressure easily wears the image carrier and the cleaning blade, thereby shortening the product life of the image carrier and the cleaning blade substantially. Consequently, the cleaning blade and the image carrier are required to be replaced more often. As a result, printing costs and occurrence of downtime due to replacement are increased and processing speed is decreased.
When irregular cleaning of the toner occurs in the above-described examples of the related-art cleaning devices, the cleaning devices are required to be replaced prematurely. As a result, similar to the related-art cleaning blade system, printing costs and occurrence of downtime due to replacement of the cleaning devices are increased and processing speed is decreased.